Method for producing aluminum-alloy shaped product, aluminum-alloy shaped product and production system

ABSTRACT

The present invention are to provide a method for producing an aluminum-alloy shaped product that exhibits high-temperature mechanical strength superior to that of a conventional aluminum-alloy forged product. 
     The present invention provides a method for producing an aluminum-alloy shaped product, comprising a step of forging a continuously cast rod of aluminum-alloy serving as a forging material, in which the aluminum-alloy contains Si in an amount of 10.5 to 13.5 mass %, Cu in an amount of 2.5 to 6 mass %, Mg in an amount of 0.3 to 1.5 mass % and Ni in an amount of 0.8 to 4%, and satisfies a relational expression of “Ni(% bymass)≧(−0.68×Cu(% by mass)+4.2(% by mass)),and heat treatment and heating steps including a step of subjecting the forging material to pre-heat treatment ( 82 ), a step ( 87 ) of heating the forging material during a course of forging of the forging material and a step of subjecting an aluminum-alloy shaped product to post-heat treatment ( 89 ), said pre-heat treatment ( 82 ) including treatment of maintaining the forging material at a temperature of −10 to 480° C. for two to six hours.

TECHNICAL FIELD

The present invention relates to a method for producing analuminum-alloy shaped product, which method includes a step of forging acontinuously cast aluminum-alloy rod serving as a forging material, toan aluminum-alloy shaped product and to a production system for theshaped product.

BACKGROUND ART

In recent years, in vehicles such as four-wheel-drive automobiles andtwo-wheel-drive automobiles (hereinafter such a vehicle will be referredto simply as an “automobile”), attempts have been made to employ analuminum-alloy forged product in an internal combustion engine piston inorder to attain high performance or to cope with environmentalregulations. This is because, when such an aluminum-alloy forged productis employed, the weight of driving parts (e.g., a piston) for aninternal combustion engine can be reduced, leading to reduction of aload upon operation of the internal combustion engine, enhancement ofoutput, or reduction of fuel consumption. Conventionally, most internalcombustion engine pistons have been produced from an aluminum-alloy castproduct. However, in the case of such a cast product, difficulty isencountered in reducing internal defects generated during the course ofcasting, and excess material must be provided on the cast product so asto ensure safety design in terms of strength. Therefore, when such acast product is employed in an internal combustion engine piston,reducing the weight of the piston is difficult.

In view of the foregoing, attempts have been made to reduce the weightof such a piston by producing the piston from an aluminum-alloy forgedproduct, in which generation of internal defects can be suppressed.

A conventional method for producing an aluminum-alloy forging materialincludes a step of preparing molten aluminum-alloy by means of a typicalsmelting technique, a step of subjecting the molten aluminum-alloy toany continuous casting technique, such as continuous casting,semi-continuous casting (DC casting) or hot top casting, to therebyproduce an aluminum-alloy cast ingot and a step of subjecting the castingot to homogenization heat treatment to thereby homogenizealuminum-alloy crystals. The thus produced aluminum-alloy forgingmaterial (cast ingot) is subjected to forging and then to a T6 treatmentof JIS (Japanese Industrial Standard) to thereby produce analuminum-alloy forged product.

JP-A 2002-294383 (Patent Document 1) discloses a method for producing a6000-series-alloy cast product, in which the homogenization treatmenttemperature is lowered or the homogenization treatment is omitted.

However, high-temperature mechanical characteristics of the cast productare not examined in Patent Document 1.

Meanwhile, the following Japanese Patent Application Publication No.2005-290545 (Patent Document 2), which is objected to produce analuminum-alloy shaped product that exhibits high-temperature mechanicalstrength superior to that of a conventional aluminum-alloy forgedproduct, discloses a method for producing an aluminum-alloy shapedproduct, comprising a step of forging a continuously cast rod ofaluminum-alloy serving as a forging material, in which thealuminum-alloy contains Si in an amount of 10.5 to 13.5 mass %, Fe in anamount of 0.15 to 0.65 mass %, Cu in an amount of 2.5 to 5.5 mass % andMg in an amount of 0.3 to 1.5 mass %, and heat treatment and heatingsteps including a step of subjecting the forging material to pre-heattreatment, a step of heating the forging material during a course offorging of the forging material and a step of subjecting a shapedproduct to post-heat treatment, the pre-heat treatment includingtreatment of maintaining the forging material at a temperature of −10 to480° C. for two to six hours.

In recent years, there has been increasing demand for an internalcombustion engine of high efficiency and high output, and accordingly,parts employed in the engine have been further required to exhibithigh-temperature mechanical strength.

Therefore, in view of the tact that an aluminum-alloy forged productenables further reduction of the weight, demand has arisen for a methodfor producing an aluminum-alloy shaped product exhibitinghigh-temperature (for example, fatigue strength at a temperature of 350°C.) mechanical strength superior to that of a conventionalaluminum-alloy forged product.

In view of the foregoing, objects of the present invention are toprovide a method for producing an aluminum-alloy shaped product thatexhibits high-temperature mechanical strength superior to that of aconventional aluminum-alloy forged product, to provide an aluminum-alloyshaped product and to provide a production system for the shapedproduct.

DISCLOSURE OF THE INVENTION

(1) In order to achieve the object, according to a first invention ofthe present invention, the present invention provides a method forproducing an aluminum-alloy shaped product, comprising a step of forginga continuously cast rod of aluminum-alloy serving as a forging material,in which the aluminum-alloy contains Si in an amount of 10.5 to 13.5mass %, Cu in an amount of 2.5 to 6 mass %, Mg in an amount of 0.3 to1.5 mass % and Ni in an amount of 0.8 to 4%, and satisfies a relationalexpression of “Ni(% by mass)≧(−0.68×Cu(% by mass)+4.2(% by mass)), andheat treatment and heating steps including a step of subjecting theforging material to pre-heat treatment, a step of preliminary heatingthe forging material before a course of forging of the forging materialand a step of subjecting a shaped product to post-heat treatment, saidpre-heat treatment including treatment of maintaining the forgingmaterial at a temperature of −10 to 480° C. for two to six hours.

(2) According to a second invention of the present invention, in thefirst mentioned method, the pre-heat treatment is performed at atemperature of at least 200° C. and 370° C. or lower.

(3) According to a third invention of the present invention, in thefirst mentioned method, the pre-heat treatment is performed at atemperature of at least −10° C. to and less than 200° C.

(4) According to a fourth invention of the present invention, in thefirst mentioned method, the pre-heat treatment is performed at atemperature of at least 370° C. and 480° C. or lower.

(5) According to a fifth invention of the present invention, in themethod according to any one of the first to fourth mentioned methods,wherein the post-heat treatment is performed at 170 to 230° C. for oneto 10 hours without performing solid solution treatment.

(6) According to a sixth invention of the present invention, in themethod according to any one of the first to fifth mentioned methods, thealuminum-alloy further contains Fe in an amount of 0.15 to 0.65 mass %.

(7) According to a seventh invention of the present invention, in themethod according to any one of the first to sixth mentioned methods, thealuminum-alloy further contains P in an amount of 0.003 to 0.02 mass %.

(8) According to an eighth invention of the present invention, in themethod according to any one of the first to seventh mentioned methods,the aluminum-alloy further contains at least one species selected fromamong Sr in an amount of 0.003 to 0.03 mass %, Sb in an amount of 0.1 to0.35 mass %, Na in an amount of 0.0005 to 0.015 mass % and Ca in anamount of 0.001 to 0.02 mass %.

(9) According to a ninth invention of the present invention, in themethod according to any one of the first to eighth mentioned methods,the aluminum-alloy further contains at least one species selected fromamong Mn in an amount of 0.1 to 1.0 mass %, Cr in an amount of 0.05 to0.5 mass %, Zr in an amount of 0.04 to 0.3 mass %, V in an amount of0.01 to 0.15 mass % and Ti in an amount of 0.01 to 0.2 mass %.

(10) According to a tenth invention of the present invention, in themethod according to any one of the first to ninth mentioned methods,during the forging step, a percent reduction of a portion of the forgingmaterial that requires high-temperature fatigue strength resistance isregulated to 90% or less.

(11) According to an eleventh invention of the present invention, in themethod according to any one of the first to tenth mentioned methods, thepreliminary heating step is performed at a temperature of 380 to 480° C.

(12) According to a twelfth invention of the present invention, in themethod according to any one of the first to eleventh mentioned methods,the continuously cast rod is produced through continuous casting of amolten alloy having an average temperature which falls within a range ofa liquidus temperature +40° C. to the liquidus temperature +230° C. at acasting speed of 80 to 2,000 mm/minute.

(13) According to a thirteenth invention of the present invention, thepresent invention further provides an aluminum-alloy shaped productproduced through the method according to any one of claims 1 to 12 andhaving a metallographic structure in which crystallization productnetworks, acicular crystallization products or crystallization productaggregates that have been formed during a course of continuous castingremain partially even after forging and heat treatment steps.

(14) According to a fourteenth invention of the present invention, thepresent invention also provides an aluminum-alloy shaped productproduced through the method according to any one of claims 1 to 12 andhaving a eutectic Si area share of 8% or more, an average eutectic Siparticle diameter of 5 μm or less, 25% ormore of eutectic Si having anacicular eutectic Si ratio of 1.4 or more, an intermetallic compoundarea share of 1.2% or more, an average intermetallic compound particlediameter of 1.5 μm or more and 30% or more of intermetallic compounds orintermetallic compound aggregates having an intermetallic compoundlength or intermetallic compound aggregate length of 3 μm or more.

(15) According to a fifteenth invention of the present invention, in thealuminum-alloy shaped product produced through the method according tothe thirteenth or fourteenth, an engine piston is made of thealuminum-alloy and includes a top surface portion and a skirt portionand the high-temperature fatigue strength of the top surface portion is50 MPa or more.

(16) According to a sixteenth invention of the present invention, Thepresent invention also provides a production system comprising acontinuous line for performing a series of steps for producing analuminum-alloy shaped product from a molten aluminum-alloy, wherein theseries of steps includes at least the steps of the method of any one ofthe first to thirteenth mentioned methods.

According to the first invention described in (1), since thealuminum-alloy includes Si, Cu, Mg, and Ni, it is possible to obtainshaped products that have excellent high-temperature fatigue strength,forgeability, ductility, and toughness. Further, since the compositionof Ni and Cu satisfies a relational expression of Ni(% bymass)≧[−0.68×Cu(% by mass)+4.2(% by mass)], it is possible to improvefatigue strength characteristics at higher temperature.

Meanwhile, conventionally, shaped products made of multilevel alloysshould be experimentally produced by changing the alloy composition, orcomplicated facilities and much time were required for the evaluation ofthe high-temperature fatigue strength. Accordingly, it was particularlydifficult to design an alloy that has fatigue strength at hightemperature.

However, it is possible to easily obtain an alloy, which has fatiguestrength characteristics at higher temperature by designing alloycomposition through using the aforementioned relational expression ofthe present invention as an index. Further, even though temperature ishigher than 350° C., it is possible to obtain aluminum-alloy shapedproducts that have excellent mechanical strength.

More specifically, for example, after aluminum-alloy shaped products areretained at a temperature of 350° C. for 100 hours, the fatigue strengththereof at a temperature of 350° C. becomes 33 MPa or more. Thesecharacteristics are characteristics required for a top surface portionof a piston of an internal combustion engine that comes in contact witha high temperature atmosphere. Accordingly, it is possible to furtherreduce the thickness of a piston of a conventional internal combustionengine by using the aluminum-alloy shaped product according to thepresent invention and to reduce the weight of a piston of an internalcombustion engine. Further, it is possible to realize to satisfy weightreduction required from the market, to reduce fuel consumption of aninternal combustion engine, and to improve output.

According to the second invention described in (2), since the heattreatment temperature of the pre-heat treatment step is in the range of200° C. to 370° C., high-temperature fatigue strength, forgeability,ductility, and toughness further become excellent, so that it ispossible to obtain better shaped products.

According to the third invention described in (3), since the heattreatment temperature of the pre-heat treatment step is in the range of−10° C. to 200° C., it is possible to obtain a shaped product havingmore excellent high-temperature fatigue strength. However, forgeability,ductility, and toughness deteriorate as compared to when the heattreatment temperature is in the range of 200° C. to 370° C.

According to the fourth invention described in (4), since the heattreatment temperature of the pre-heat treatment step is in the range of370° C. to 480° C., it is possible to obtain a shaped product havingmore excellent forgeability, ductility, and toughness. However,high-temperature fatigue strength deteriorates as compared to when theheat treatment temperature is in the range of 200° C. to 370° C.

According to the fifth invention described in (5), the forging materialis retained at a temperature of 170° C. to 230° C. for 1 to 10 hours,without performing a solid solution treatment at a post-heat treatmentstep. Accordingly, it is possible to obtain a shaped product having moreexcellent high-temperature fatigue strength. However, ductility andtoughness deteriorate as compared to when a solution treatment isperformed and the forging material is retained at a temperature of 170°C. to 230° C. for 1 to 10 hours.

According to the sixth invention described in (6), since thealuminum-alloy includes 0.15 to 0.65% by mass of Fe, Al—Fe, Al—Fe—Si, orAl—Ni—Fe based particles are crystallized, thereby improvinghigh-temperature mechanical strength. Further, the content of 0.15 to0.65% by mass of Fe suppresses the increase of the large crystallizationproducts and improves forgeability, high-temperature fatigue strength,and toughness.

According to the seventh invention described in (7), the aluminum-alloyincludes 0.003 to 0.02% by mass of P. Since generating primary Sicrystals, P is preferable when wear resistance is a priority. Inaddition, P has an effect of micronizing primary Si crystals, and actsby suppressing the decrease of forgeability, ductility, orhigh-temperature fatigue strength that is caused by primary Si crystalsgenerated. Further, the content of 0.003 to 0.02% by mass of Psuppresses the increase of large primary Si crystals, thereby improvingforgeability, high-temperature fatigue strength, and toughness.

According to the eighth invention described in (8), the aluminum-alloymay include one or the combination of two or more of 0.003 to 0.03% bymass of Sr, 0.1 to 0.35% by mass of Sb, 0.0005 to 0.015% bymass of Na,and 0.001 to 0.02% bymass of Ca. Accordingly, it is possible to suppressthe generation of primary Si crystals and this is preferable whenforgeability, ductility, and toughness are priorities. Further, thecontents of Sr, Sb, Na, and Ca in this range suppress the generation ofprimary Si crystals, and improve forgeability, toughness, andhigh-temperature fatigue strength.

According to the ninth invention described in (9), the aluminum-alloymay include one or the combination of two or more of 0.1 to 1.0% by massof Mn, 0.05 to 0.5% by mass of Cr, 0.04 to 0.3% by mass of Zr, 0,01 to0.15% by mass of V, and 0.01 to 0.2% by mass of Ti. Accordingly, Al—Mn,Al—Fe—Mn—Si, Al—Cr, Al—Fe—Cr—Si, Al—Zr, Al—V, and Al—Ti based compoundsare crystallized or precipitated, thereby improving high-temperaturemechanical strength of the aluminum-alloy. Further, the contents of Mn,Cr, Zr, V, and Ti in this range suppress the increase of largecrystallization products, and improve forgeability, high-temperaturefatigue strength, and toughness.

According to the tenth invention described in (10), since a percentreduction of a portion requiring high-temperature fatigue resistantstrength is 90% or less in the forging step, the networks, acicularcrystallization products, or aggregates of the crystallization productsare appropriately divided and remain. Therefore, it is possible toobtain shaped products that have excellent ductility, toughness, andhigh-temperature fatigue strength.

According to the eleventh invention described in (11), since apreliminary heating temperature before processing is in the range of380° C. to 480° C. in the forging step, it is possible to obtain shapedproducts that have excellent high-temperature fatigue strength,forgeability, ductility, and toughness.

According to the twelfth invention described in (12), the continuouslycast rod is obtained by casting an aluminum-alloy, of which an averagetemperature of the molten alloy corresponds to a liquidus line of +40°C. to +230° C., at a casting speed of 80 (mm/min) to 2000 (mm/min) by acontinuous casting methods Accordingly, it is possible to obtain thenetworks, acicular crystallization products, or aggregates of theuniform and fine crystallization products, and to obtain shaped productsthat have excellent high-temperature fatigue strength, forgeability,ductility, and toughness.

According to the thirteenth invention described in (13), networks ofcrystallization products, acicular crystallization products, oraggregates of crystallization products formed during continuous castingpartially remain in the structure even after forming and a heattreatment. Accordingly, it is possible to obtain shaped products thathave excellent high-temperature fatigue strength, forgeability,ductility, and toughness.

According to the fourteenth invention described in (14), a sample havingan area occupation ratio of eutectic Si of 8% or more, an average grainsize of eutectic Si of 5 μm or less, and an acicular eutectic Si ratioof eutectic Si of 1.4 or more corresponds to 25% or more; and a samplehaving an area occupation ratio of an intermetallic compound of 1.2% ormore, an average grain size of an intermetallic compound of 1.5 μm ormore, and a length of an intermetallic compound or a length of anaggregate of a contacted intermetallic compound is 3 μm or morecorresponds 30% or more. Accordingly, it is possible to reliably obtainshaped products that have excellent high-temperature fatigue strength,forgeability, ductility, and toughness.

According to the fifteenth invention disclosed in (15), since thehigh-temperature fatigue strength of the top surface portion is 50 MPaor more, the shaped products have sufficient high-temperature fatiguestrength and may be suitably used for a top surface portion, and thelike, of a piston of an internal combustion engine.

According to the sixteenth invention described in (16), a series ofsteps between molten metal and the aluminum-alloy shaped product arebuilt up as a continuous line, and any one of the above-mentionedmethods for production of aluminum-alloy shaped product is necessarilyincluded in the series of steps. Accordingly, it is possible to improvefatigue strength characteristics at higher temperature.

Meanwhile, conventionally, shaped products made of multilevel alloysshould be experimentally produced by changing the alloy composition, orcomplicated facilities and much time were required for the evaluation ofthe high-temperature fatigue strength. Accordingly, it was difficult todesign an alloy that has fatigue strength at particularly hightemperature.

However, it is possible to easily obtain an alloy, which has fatiguestrength characteristics at higher temperature by designing alloycomposition by using the relational expression of the present inventionas an index. Further, even though temperature is higher than 350° C., itis possible to obtain aluminum-alloy shaped products that have excellentmechanical strength.

More specifically, for example, after aluminum-alloy shaped products areretained at a temperature of 350° C. for 100 hours, the fatigue strengththereof at a temperature of 350° C. becomes 33 MPa or more. Thesecharacteristics are, for example, characteristics required for a topsurface portion of a piston of an internal combustion engine that comesin contact with a high temperature atmosphere. Accordingly, it ispossible to further reduce the thickness of a piston of a conventionalinternal combustion engine by using the aluminum-alloy shaped productaccording to the present invention and to reduce the weight of a pistonof an internal combustion engine. Further, it is possible to satisfyweight reduction required from the market, and realize to reduce fuelconsumption of an internal combustion engine, and to improve output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a forging production system that is an exampleof a production line for realizing production method according to thepresent invention;

FIG. 2 is a view showing an example of a portion near a mold of acontinuous casting apparatus that is used in the present invention;

FIG. 3 is a view showing another example of the portion near the mold ofthe continuous casting apparatus that is used in the present invention;

FIG. 4 is a view showing the effective mold length of the continuouscasting apparatus that is used in the present invention;

FIG. 5 is a view showing another example of the continuous castingapparatus that is used in the present invention;

FIG. 6 is a view illustrating a relationship between contents of Ni andCu that are in an aluminum-alloy;

FIG. 7A is a plan view of a piston having the shape of Examples 17 and18 of the present invention and Comparative Examples 11 to 13;

FIG. 7B is a front view of the piston shown in FIG. 7A; and

FIG. 8 is a cross-sectional view taken along line VIII-VIII of FIG. 7A.

BEST MODE FOR CARRYING OUT THE INVENTION

The alloy composition of the shaped product according to the presentinvention will be described.

A molten aluminum-alloy used in the present invention includes 10.5 to13.5% by mass (preferably, 11.5 to 13% by mass) of Si, 2.5 to 6% by mass(preferably, 3.5 to 5.5% by mass) of Cu, 0.3 to 1.5% by mass(preferably, 0.5 to 1.3% by mass) of Mg, and 0.8 to 4% by mass(preferably, 1.8 to 3.5% by mass) of Ni, and is adjusted to havecomposition that satisfies a relational expression of Ni(% bymass)≧[−0.68×Cu(% by mass)+AA(% by mass)] (wherein, AA is a constant andAA≧4.2 preferably AA≧4.7).

Si increases high-temperature mechanical strength and wear resistance bythe distribution of eutectic Si, and coexists with Mg and precipitatesMg₂Si particles, thereby improving high-temperature mechanical strength.If Si content is less than 10.5% bymass, the above-mentioned effects aresmall. If Si content exceeds 13.5% by mass, a large amount of primary Sicrystals is crystallized, so that high-temperature fatigue strength,ductility, and toughness are decreased.

Ni generates Al—Ni based and Al—Ni—Cu based crystallization products,and improves high-temperature mechanical strength by using thecrystallization products. If Ni content is less than 0.8% bymass, theabove-mentionedeffects are small. If Ni content exceeds 4% by mass, theamount of large crystallization products is increased, so thatforgeability or high-temperature fatigue strength, ductility, andtoughness are decreased.

Cu precipitates CuAl₂ particles, and generates Al—Cu based and Al—Ni—Cubased crystallization products, thereby improving high-temperaturemechanical strength. If Cu content is less than 2.5% bymass, theabove-mentionedeffects are small. If Cu content exceeds 6% bymass, theamount of large Al—Cu basedcrystallization products is increased, sothat forgeability or high-temperature fatigue strength, ductility, andtoughness are decreased.

Mg coexists with Si and precipitates Mg₂Si particles, thereby improvinghigh-temperature mechanical strength. If Mg content is less than 0.3% bymass, the above-mentioned effects are small. If Mg content exceeds 1.5%by mass, the amount of large Mg₂Si crystallization products isincreased, so that forgeability or high-temperature fatigue strength,ductility, and toughness are decreased.

Further, in the present invention, the composition of Ni and Cu needs tosatisfy a relational expression of Ni(% by mass)≧[−0.68×Cu(%bymass)+AA(% by mass)] (wherein, AA is aconstant and AA≧4.2 preferablyAA≧4.7). The reason for this is that a fatigue strength characteristicat higher temperature is improved if Ni and Cu satisfy this relationalexpression. Meanwhile, since having a large amount of a generatednetwork-like or acicular intermetallic compounds that contribute tohigh-temperature strength, the aluminum-alloy shaped product that areprepared to have a constant AA equal to or larger than 4.7 arepreferable.

The mechanism of the improvement of the fatigue strength characteristicis not clear, but may be estimated as follows. It is considered thatAl—Ni based crystallization products, Al—Ni—Cu based crystallizationproducts, Al—Cu based crystallization products, and Co dissolved in analuminum matrix under high-temperature environment contribute most tothe improvement of high-temperature mechanical strength. A relationshipbetween Cu content and Ni content where high-temperature mechanicalstrength is effectively improved by these crystallization products andthe solid solution of Cu has been deduced from the above-mentionedrelational expression.

The fatigue strength of the shaped product using the aluminum-alloy at atemperature of 350° C. is equal to or higher than 33 MPa that is apreferable value, more preferably, 43 MPa.

Further, the fatigue strength of the shaped product using thealuminum-alloy at a temperature of 300° C. is equal to or higher than 55MPa.

It is preferable that the molten alloy contain one or two or more of 0.1to 1% by mass (preferably, 0.2 to 0.5% by mass). of Mn, 0.05 to 0.5% bymass (preferably, 0.1 to 0.3% by mass) of Cr, 0.04 to 0.3% by mass(preferably, 0.1 to 0.2% by mass) of Zr, and 0.01 to 0.15% by mass(preferably, 0.05 to 0.1% by mass) of V, and 0.01 to 0.2% by mass(preferably, 0.02% to 0.1% by mass) of Ti. The reason why Mn, Cr, Zr, V,and Ti is contained is to crystallize or precipitate Al—Mn orAl—Fe—Mn—Si based compounds, Al—Cr or Al—Fe—Cr—Si based compounds, Al—Zrbased compounds, Al—V based compounds, and Al—Ti based compounds, and toimprove the high-temperature mechanical strength of the aluminum-alloy.If Mn content is less than 0.1% by mass, Cr content is less than 0.05%by mass, Zr content is less than 0.04% by mass, V content is less than0.01% by mass, and Ti content is less than 0.01% by mass,theabove-mentioned effects aresmall. If Mn content exceeds 1.0% by mass,Cr content exceeds 0.5% by mass, Zr content exceeds 0.3% by mass, Vcontent exceeds 0.15% by mass, and Ti content exceeds 0.2% by mass, theamount of large crystallization products is increased, so thatforgeability, high-temperature fatiguestrength, and toughness aredecreased.

Further, it ispreferable that the molten alloy include 0.15 to 0.65%bymass (preferably, 0.3 to 0.5% bymass) of Fe, and Al—Fe, Al—Fe—Si, orAl—Ni—Fe based particles are crystallized, thereby improvinghigh-temperature mechanical strength. If Fe content is less than 0.15%by mass, the above-mentioned effects are small. If Fe content exceeds0.65% by mass, the amount of Al—Fe, Al—Fe—Si, or Al—Ni—Fe based largecrystallization products is increased, so that forgeability orhigh-temperature fatigue strength, ductility, and toughness aredecreased.

Furthermore, it is preferable that the molten alloy includes 0.003 to0.02% by mass (preferably, 0.007 to 0.016% by mass) of P. Sincegenerating primary Si crystals, P is preferable when wear resistance isa priority. In addition, P has an effect of micronizing primary Sicrystals, and suppresses the decrease of forgeability, ductility, orhigh-temperature fatigue strength that is caused by primary Si crystalsgenerated. If P content is less than 0.003% by mass, the effect ofmicronizing primary Si crystals is small, large primary Si crystals isgenerated at the center of an ingot, and forgeability orhigh-temperature fatigue strength, ductility, and toughness aredecreased. If P content exceeds 0.02% by mass, the amount of generatedprimary Si crystals is increased, and forgeability or high-temperaturefatigue strength, ductility, and toughness are decreased.

In addition, the molten alloy contains one or two or more of 0.003 to0.03% by mass (preferably, 0.01 to 0.02% by mass) of Sr, 0.1 to 0.35% bymass (preferably, 0.15 to 0.25% by mass) of Sb, 0.0005 to 0.015% by mass(preferably, 0.001 to 0.01% by mass) of Na, and 0.001 to 0.02% by mass(preferably, 0.005 to 0.01% by mass) of Ca, which is preferable becausethere is an effect of micronizingprimary Si crystals. If Sr content isless than 0.003% by mass, Sb content is less than 0.1% by mass, Nacontent is less than 0.0005% by mass, and Ca content is less than 0.001%by mass, theabove-mentioned effects aresmall. If Sr contentexceeds 0.03%by mass, Sb content exceeds 0.35% by mass, Na content exceeds 0.015% bymass, and Ca content exceeds 0.02% by mass, the amount of largecrystallization products is increased or casting defects are generated,so that forgeability, high-temperature fatigue strength, and toughnessare decreased.

The composition ratios of the aluminum-alloy shaped product and an alloyingredient of an ingot can be confirmed by a method using, for example,an optical emission spectrometer (e g., PDA-5500, product of ShimadzuCorporation), which is based on photoelectric that is disclosed in JISH1305.

An embodiment of the present invention will be described in detail belowwith reference to drawings.

FIG. 1 is a view showing a production system that is an example of aproduction line for realizing production method according to the presentinvention. In FIG. 1, a forging production system configures acontinuous casting apparatus 81 that horizontally casts a continuouslycast rod from molten metal and cuts the continuously cast rod to apredetermined length; a pre-heat treatment apparatus 82 for performing aheat treatment on the continuously cast rod that is cast by thecontinuous casting apparatus 81; a correction apparatus 83 forcorrecting the bend of the continuously cast rod if the continuouslycast rod heat-treated by the pre-heat treatment apparatus 82 is bent; apeeling apparatus 84 for removing the outer peripheral portion of thecontinuously cast rod of which the bent is corrected by the correctionapparatus 83; a cutting apparatus 85 for cutting the continuously castrod of which the outer peripheral portion is removed by the peelingapparatus 84 into cut pieces that have a length required for the forgingof the shaped product; an upsetting apparatus (not shown) thatpreliminarily heats the cut pieces cut by the cutting apparatus 85 andupsets the cut pieces; lubrication apparatuses 86A and 86B for applyinga graphite lubricant to the preliminarily heated forging material, forimmersing the preliminarily heated forging material in a graphitelubricant, or for coating the preliminarily heated forging material witha graphite lubricant in order to coat the forging material which isupset by the upsetting apparatus with a lubricant; a forging apparatus88 for forging the product (preform) from the forging material that isfurther heated by the preliminary heating apparatus 87 and coated with alubricant; and a post-heat treatment apparatus 89 for performing apost-heat treatment on the forged products (product) that are forged bythe forging apparatus 88

For example, the post-heat treatment apparatus 89 may configure a solidsolution treatment apparatus 90 that performs a solution treatment onthe forged products, a quenching apparatus 91 that quenches the productheated by the solid solution treatment apparatus 90, and an agingtreatment apparatus 92 that performs an aging treatment on the productquenched by the quenching apparatus 91. If the solution treatment isomitted, it is preferable that the aging treatment apparatus 92 beprovided behind the forging apparatus 88 without providing the solidsolution treatment apparatus 90 and the quenching apparatus 91.

Meanwhile, the peeling apparatus 84 and the upsetting apparatus may beomitted. Further, the conveyance between the apparatus may be achievedby automatic conveying apparatuses. Further, a lubricant coatingtreatment of the lubrication apparatuses 86A and 86B may be substitutedwith an apparatus 86C for bonde treatment (phosphoric-acid-salt coatingtreatment).

In this case, the pre-heat treatment apparatus 82 has a function toretain the temperature of the forging material in the range of −10° C.to 480° C. for 2 to 6 hours. The preliminary heating apparatus 87 has afunction to make the temperature of the forging material in the range of380° C. to 480° C. The solid solution treatment apparatus 90 and thequenching apparatus 91 of the post-heat treatment apparatus 89 havefunctions to make the temperature of the forged products (shapedproducts) for the solution be in the range of 480° C. to 520° C. andthen to quench the forged products. The aging treatment apparatus 92 ofthe post-heat treatment apparatus 89 has a function to retain thetemperature of the forged products (shaped products) in the range of170° C. to 230° C.

A method for production used in the production system according to thepresent invention includes a step of performing a pre-heat treatment onthe round rod that is obtained by casting an aluminum-alloy by acontinuous casting method, a step of forming the preform from pre-heattreated materials as forging material by hot plastic forming, and a stepof performing a post-heat treatment after the plastic forming. Thetemperature of the pre-heat treatment is in the range of −10° C. to 480°C., and the temperature of the forging material during the hot plasticforming is in the range of 380° to 480° C. In the post-heat treatmentstep, solution heating is performed so that the temperature of thepreform is in the range of 480° C. to 520° C., or temperature isdirectly managed so as to satisfy a temperature condition of 170° C. to230° C. without performing the solution treatment. Accordingly, shapedproducts are consistently produced by performing steps that include fromthe casting step to each of all heat treatment steps. As a result, it ispossible to stably produce shaped products having preferred mechanicalstrength.

Forging may be mentioned to be used as the above-mentioned plasticforming. However, as long as the temperature of the pre-heat treatment,the conditions of the temperature of the forging material during the hotplastic forming, and the temperature of the post-heat treatment aresatisfied, the combination of rolling working and extruding working maybe used as the method for production according to the present invention.The reason for this is that it is possible to obtain an effect of thepresent invention in controlling the network of the structure orcrystallization products in either case.

The aluminum-alloy shaped product according to the present invention maybe suitably used as parts that require mechanical strength at hightemperature. Accordingly, the shaped product having the shapes of, forexample, an engine piston, a valve litter, a valve retainer, a cylinderliner, and the like, may be produced according to the present invention;and the shaped product may be formed in desired shapes by furtherperforming machining on the shaped product with a lathe, a machiningcenter, and the like, if necessary, so as to be used as parts forvarious products.

Any one of a known hot top continuous casting, a known verticalcontinuous casting, a known horizontal continuous casting, and a knownDC casting may be used in a part of a basic solidification method of themethod for production that is used in the present invention. Forexample, the method may be a horizontal continuous casting that suppliesone or two or more fluids, which are selected from a gas lubricant and aliquid lubricant, and the gas obtained through thermal decomposition ofthe liquid thereof, onto the inner wall surfaces of a tubular mold thathas forced cooling and is held so as to have a central axis parallel toa horizontal direction; supplies a molten aluminum-alloy containing Sito one end of the tubular mold so as to form columnar molten alloy; anddraws an ingot which is formed by solidifying the columnar molten alloyin the tubular mold from the other end of the tubular mold. A case wherethe present invention is applied to a horizontal continuous casting willbe described below.

FIG. 2 is a view showing an example of a portion near the mold of thecontinuous casting apparatus that is used in the present invention. Atundish 250, a refractory plate-like body 210, and a tubular mold 201are disposed so that an molten alloy 255 stored in the tundish 250 issupplied to the tubular mold 201 through the refractory plate-like body210. The tubular mold 201 is held so that a center axis 220 of the moldis substantially parallel to a horizontal direction. A means forforcedly cooling the mold is disposed in the tubular mold 201 and ameans for forcedly cooling the mold for a cast ingot 216 is disposed atan outlet of the tubular mold 201 so that the molten alloy 255 becomesthe cast ingot 216. In FIG. 2, a cooling water showering apparatus 205is provided as an example of a means for forcedly cooling the cast ingot216. A drive apparatus (not shown) is disposed near the outlet of thetubular mold 201 so that the forcedly cooled and cast ingot 216 is drawnat a constant speed and continuously cast. Further, a synchronizedcutting machine (not shown), which cuts the drawn and cast rod to apredetermined length, is provided.

Another example of the portion near the mold of the continuous castingapparatus, which is used in the present invention, will be describedwith reference to FIG. 3. FIG. 3 is a schematic cross-sectional view ofan example of a DC casting apparatus. In the DC casting apparatus, amolten aluminum-alloy 1 is introduced into a stationary water-coolingmold 5, which is made of an aluminum-alloy or copper, through a trough2, a dip tube 3, and a floating distributor 4. The water-cooling mold 5is cooled by cooling water 5A. A molten aluminum-alloy 6 introduced intothe water-cooling mold 5 forms a solidification shell 7 at a portionthereof, which comes in contact with the water-cooling mold 5, and isconstructed. A solidified aluminum-alloy ingot 7A is drawn downward fromthe water-cooling mold 5 by a lower mold 9. In this case, thealuminum-alloy ingot 7A is further cooled by cooling water jet 8 that issupplied from the water-cooling mold 5, thereby being completelysolidified. If the lower mold 9 reaches a lower end where the lower mold9 can be moved, the aluminum-alloy ingot 7A is cut at a predeterminedposition and withdrawn.

Referring to FIG. 2, the tubular mold 201 is held so that the centeraxis 220 of the mold is substantially parallel to a horizontaldirection. The tubular mold 201 includes a means for forcedly coolingthe tubular mold 201. This means for forcedly cooling the mold 201 coolsthe wall surfaces of the mold by cooling water 202 that is stored in amold's cooling water cavity 204; removes the heat of the columnar moltenalloy 215, which is tilled in the tubular mold 201, from the surfaces ofthe molten metal that comes in contact with the inner wall of the mold201; and forms a solidification shell on the surface of the moltenmetal. The tubular mold 201 further includes a means for forcedlycooling the mold. This means for forcedly cooling the mold dischargescooling water from the cooling water showering apparatus 205 so thatcooling water comes in direct contact with the cast ingot 216 at the endof the outlet of the tubular mold 201, thereby solidifying the columnarmolten alloy 215 stored in the tubular mold 210. In addition, an end ofthe tubular mold 201, which is positioned opposite to nozzles of thecooling water showering apparatus 205, is connected to the tundish 250through the refractory plate-like body 210.

In FIG. 2, cooling water that is used to forcedly cool the tubular mold201, and cooling water that is used to forcedly cool the cast ingot 216are supplied through a cooling water feed tube 203. However, the coolingwater may be separately supplied.

A distance from a position, where the extension line of the central axisof the nozzle of the cooling water showering apparatus 205 intersectsthe surface of the cast ingot 216, to the contact surface between thetubular mold 201 and the refractory plate-like body 210 is referred toas an effective mold length (see reference numeral L of FIG. 4). It ispreferable that the effective mold length be in the range of 15 to70 mm.If the effective mold length is less than 15 mm, such as a good film isnot formed, so that casting cannot be performed. If the effective moldlength exceeds 70 mm, forced cooling is ineffective and thesolidification caused by the inner wall of the mold is dominant.Accordingly, the contact resistance between the tubular mold 201 and thecolumnar molten alloy 215 or the solidification shell is increased, sothat cracks are generated on the casting surface or the tubular mold 201is torn off therein, and the like. Therefore, this is not preferable dueto unstable casting.

It is preferable that a material of the tubular mold 201 be one or thecombination of two or more selected from aluminum, copper, or alloysthereof. The combination of materials may be selected in considerationof thermal conductivity, heat resistance, and mechanical strength.

Further, it is preferable in the mold that a permeable porous member 222having a self-lubricity be provided in a ring shape on the surface ofthe tubular mold 201 coming in contact with the columnar molten alloy215. The ring shape means that the permeable porous member is providedon the entire inner wall 221 of the tubular mold 201 in acircumferential direction. The air permeability of the permeable porousmember 222 may be in the range of 0.005 to 0.03 [L(liter)/(cm²/min)],more preferably, 0.07 to 0.02 [L/(cm²/min)]. The thickness of thepermeable porous member 222 to be provided is not particularly limited,but is preferably in the range of 2 to 10 mm, more preferably, 3 to 8mm. For example, graphite of which air permeability is in the range of0.008 to 0.012 ]L/(cm²/min)] may be used as the permeable porous member222. In this case, the air permeability is obtained by measuring theamount of air, which has a pressure of 2 kg/cm² and is ventilatedthrough a test piece having a thickness of 5 mm, per minute under.

It is preferable to use a tubular mold 201 in which a permeable porousmember 222 is provided in the range of 5 to 15 mm within the range ofthe effective mold length. It is preferable that an O-ring 213 isprovided on the matching surface of the tubular mold 201, the refractoryplate-like body 210, and the permeable porous member 222.

The shape of the inner wall 221 of the radial cross-section of thetubular mold 201 may have a triangular shape, a rectangular shape, or anirregular shape having no symmetry axis nor symmetry plane, in additionto a circular shape. Alternatively, a core may be provided in the moldin order to form a hollow cast ingot. Further, the tubular mold 201 is atubular mold of which both ends are opened. The molten alloy 255 issupplied into the tubular mold 201 from one end of the tubular mold 201through a molten alloy inlet 211 that is formed through the refractoryplate-like body 210, and the cast ingot 216 is extruded or drawn fromthe other end of the tubular mold 201.

The inner wall 221 of the tubular mold 201 is formed to have anelevation angle in the range of 0 to 3°, more preferably, 0 to 1° withrespect to the center axis 220 of the mold in a direction where the castingot 216 is drawn. If the elevation angle is less than 0°, resistanceis applied to the outlet of the tubular mold 201 when the cast ingot 216is drawn from the tubular mold 201. For this reason, casting cannot beperformed. Meanwhile, if the elevation angle exceeds 3°, the inner wall221 of the tubular mold 201 comes in insufficient contact with thecolumnar molten alloy 215. Accordingly, an effect of removing heat thatheat is removed from the columnar molten alloy 215 or the solidificationshell to the tubular mold 201 deteriorates, so that solidificationbecomes insufficient. As a result, this is not preferable due to theincrease of the possibility of casting troubles that re-melted surfaceis formed on the surface of the cast ingot 216 or the molten alloy 255,which is unsolidified, is discharged from the end of the tubular mold201, and the like.

The tundish 250 configures a molten alloy receiving inlet 251 forreceiving a molten aluminum-alloy that is adjusted to have prescribedalloy ingredients by an external melting furnace or the like, a moltenalloy reservoir 252, and an outlet 253 that makes the molten metal toflow into the tubular mold 201. The tundish 250 maintains the level 254of the molten alloy 255 at a position that is higher than the uppersurface of the tubular mold 201, and stably distributes the molten alloy255 to each tubular mold 201 in the case of multiple casting. The moltenalloy 255 held in the molten alloy reservoir 252 of the tundish 250 ispoured in the tubular mold 201 from the molten alloy inlet 211 that isprovided through the refractory plate-like body 210.

The refractory plate-like body 210 is used to isolate the tundish 250from the tubular mold 201, and can be produced from a material havingrefractory heat-insulating properties. For example, Lumiboardmanufactured by NICHIAS Corporation, INSURAL manufactured by FOSECOJAPAN, Ltd., or Fiber Blanket Board manufactured by IBIDEN CO., LTD. maybe used as the refractory plate-like body. The refractory plate-likebody 210 has the shape that can form the molten alloy inlet 211. One ormore pouring ports 211 may be formed at a portion of which therefractory plate-like body 210 protrudes inward from the inner wall 221of the tubular mold 201.

Reference numeral 208 denotes a fluid feed-tube through a fluid issupplied. A lubrication fluid may be used as the fluid. The fluid may beone kind or two kinds or more selected from gaseous lubricants andliquid lubricants. It is preferable that supply pipes for a gaseouslubricant and a liquid lubricant be separately provided.

The fluid, which is pressurized and supplied from the fluid feed-tube208, is supplied to a gap, which is formed between the tubular mold 201and the refractory plate-like body 210, through a circular path 224. Itis preferable that a gap of 200 μm or less be formed to the portionbetween the tubular mold 201 and the refractory plate-like body 210. Thegap has a size so that the molten alloy 255 can not permeate through thegap and the fluid can flow to the inner wall 221 of the tubular mold201. In the mode shown in FIG. 2, the circular path 224 is formed on theouter peripheral surface of the permeable porous member 222 that isprovided on the tubular mold 201. The fluid permeates into the permeableporous member 222 due to applied pressure, is fed onto the entiresurface of the permeable porous member 222 that comes in contact withthe columnar molten alloy 215, and is supplied onto the inner wall 221of the tubular mold 201. The liquid lubricant may be heated and changedinto decomposed gas, and may be supplied onto the inner wall 221 of thetubular mold 201.

As a result, it is possible to improve the lubrication between thepermeable porous surfaces of the tubular mold 201, and the periphery ofthe columnar molten alloy 215 and the periphery of the solidificationshell. The permeable porous member 222 is provided in a ring shape, sothat it is possible to obtain a better lubrication effect and to easilycast a continuously cast rod made of an aluminum-alloy.

A corner space 230 is formed by one or two or more selected from thesupplied gases, the supplied liquid lubricant, and the gases decomposedfrom the liquid lubricant.

A casting step included in the method for production according to thepresent invention will be described.

In FIG. 2, the molten alloy 255 stored in the tundish 250 is supplied tothe tubular mold 201, which is held so as to have a center axis 220 ofthe mold substantially parallel to a horizontal direction, through therefractory plate-like body 210. The molten alloy is forcedly cooled atthe outlet of the tubular mold 201, and becomes the cast ingot 216.Since the cast ingot 216 is drawn at a constant speed by a driveapparatus that is provided near the outlet of the tubular mold 201, themolten alloy is continuously cast into a cast rod. The drawn cast rod iscut to a predetermined length by a synchronized cutting machine. Thatis, an aluminum-alloy, of which the average temperature of a moltenalloy 255 corresponds to a liquidus line of +40° C. to +230  C. can becast into the continuously cast rod at a casting speed of 300 (mm/min)to 2000 (mm/min) by a continuous casting method. Under this condition,it is possible to obtain shaped products where crystallization productsare finely dispersed and forgeability and high-temperature mechanicalstrength are excellent. It is preferable that a casting speed be in therange of 80 (mm/min) to 400 (mm/min) in case of a hot top continuouscasting, a vertical continuous casting, and a DC casting. Accordingly,it is preferable that a casting speed be in the range of 80 (mm/min) to2000 (mm/min).

The composition of the molten aluminum-alloy 255 stored in the tundish250 will be described.

The molten alloy 255 includes 10.5 to 13.5% by mass (preferably, 11.5 to13% by mass) of Si, 2.5 to 6% by mass (preferably, 3.5 to 5.5% by mass)of Cu, 0.3 to 1.5% by mass (preferably, 0.5 to 1.3% by mass) of Mg, and0.8 to 4% by mass (preferably, 1.8 to 3.5% by mass) of Ni, and is analuminum-alloy that satisfies a relational expression of Ni(% bymass)≧[−0.68×Cu(% by mass)+AA(% by mass)] (wherein, AA is a constant andAA≧4.2 preferably AA≧4.7 is satisfied.).

It is preferable that the molten alloy 255 contain one or two or more of0.1 to 1% by mass (preferably, 0.2 to 0.5% by mass) of Mn, 0.05 to 0.5%by mass (preferably, 0.1 to 0.3% by mass) of Cr, 0.04 to 0.3% by mass(preferably, 0.1 to 0.2% by mass) of Zr, and 0.01 to 0.15% by mass(preferably, 0.05 to 0.1% by mass) of V, and 0.01 to 0.2% by mass(preferably, 0.02% to 0.1% by mass) of Ti.

Further, it is preferable that the molten alloy includes 0.15 to 0.65%by mass (preferably, 0.3 to 0.5% by mass) of Fe.

Furthermore, it is preferable that the molten alloy includes 0.003 to0.02% by mass (preferably, 0.007 to 0.016% by mass) of P.

In addition, the molten alloy contains one or two or more of 0.003 to0.03% by mass (preferably, 0.01 to 0.02% by mass) of Sr, 0.1 to 0.35% bymass (preferably, 0.15 to 0.25% by mass) of Sb, 0.0005 to 0.015% by mass(preferably, 0.001 to 0.01% by mass) of Na, and 0.001 to 0.02% by mass(preferably, 0.005 to 0.01% by mass) of Ca, which is preferable becausethere is an effect of micronizing eutectic Si crystals.

A difference between the height of the level 254 of the molten alloy 255that is stored in the tundish 250, and the height of the upper surfaceof the inner wall 221 of the tubular mold 201 is set in the range of 0to 250 mm, more preferably, 50 to 170 mm. If the difference is providedto both, the pressure of the molten alloy 255 supplied inside thetubular mold 201, liquid lubricant, and gas obtained from thevaporization of the liquid lubricant are suitably balanced with eachother. The reason for this is that the castability is stabilized and itis possible to easily produce a continuously cast rod made of analuminum-alloy. If level sensors, which are used to measure and monitorthe height of the level 254 of the molten alloy 255, are provided to thetundish 250, it is possible to accurately manage the difference andmaintain the difference at a predetermined value.

Vegetable oil, which is liquid lubricant, may be used as the liquidlubricant. For example, rape seed oil, castor oil, and salad oil maybeused as the liquid lubricant. Since hardly having an adverse effect onenvironment, these are preferable.

It is preferable that the amount of supplied liquid lubricant be in therange of 0.05 (mL/min) to 5 (mL/min) [more preferably, 0.1 (mL/min) to 1(mL/min)]. If the amount of supplied liquid lubricant is excessivelysmall, the breakout of an ingot is generated due to the lack oflubrication. If the amount of supplied liquid lubricant is excessivelylarge, surplus oil will be mixed to the ingot. For this reason, there isa concern that the formation of crystal grains having a uniform sizewill deteriorate.

It is preferable that the casting speed, that is, a speed where the castingot 216 is drawn from the tubular mold 201, be in the range of 300(mm/min) to 2000 (mm/min) [more preferably, 600 (mm/min) to 2000(mm/min)]. This is preferable because the networks of thecrystallization products formed by casting become uniform and fine andresistance against the deformation of an aluminum matrix at hightemperature is increased, and high-temperature mechanical strength isimproved. Of course, the effect of the present invention is not limitedby the casting speed. However, if the casting speed is increased, theeffect thereof becomes significant.

It is preferable that the amount of the cooling water discharged fromthe cooling water showering apparatus 205 be in the range of 5 (L/min)to 30 (L/min) [more preferably, 25 (L/min) to 30 (L/min)] per mold. Ifthe amount of cooling water is excessively small, the breakout will begenerated or the surface of the cast ingot 216 will be re-melted, sothat non-uniform structure will be formed. For this reason, there is aconcern that the formation of crystal grains having a uniform size willdeteriorate. Meanwhile, if the amount of cooling water is excessivelylarge, a very large amount of heat will be removed from the tubular mold201, so that casting cannot be performed. Of course, the effect of thepresent invention is not limited by the amount of cooling water.However, if the cooling capacity is increased to increase a temperaturegradient from a solidification interface to the interior of the tubularmold 201, the effect thereof becomes significant.

It is preferable that the average temperature of the molten alloy 255,which flows into the tubular mold 201 from the tundish 250, correspondto aliquidus line of +40° C. to +230° C. (more preferably, a liquidusline of +60 to +200° C.). If the temperature of the molten alloy 255 isexcessively low, large crystallization products will be formed in thetubular mold 201 and before that. For this reason, there is a concernthat the formation of crystal grains having a uniform size deteriorates.Meanwhile, if the temperature of the molten alloy 255 is high, a largeamount of hydrogen gas will be included in the molten alloy 255 and alsoinclude porosities in the cast ingot 216. For this reason, there is aconcern that the formation of crystal grains having a uniform size willdeteriorate.

In the present invention, these casting conditions are controlled sothat eutectic Si of the structure of the castings or intermetalliccompounds become the networks of the crystallization products, acicularcrystallization products, or aggregates of crystallization productsformed during the continuous casting, with few spherical aggregates.Accordingly, the effect of each of subsequent heat treatments becomeseffective, which is preferable.

In the present invention, as a pre-heat treatment, it is important thata cast rod after having been cast is retained in the temperature rangeof −10° C. to 480° C. (preferably, −10° C. to 370° C.) for 2 to 6 hoursbefore being provided to a forging step as forging material. It is morepreferable that the temperature condition corresponds to roomtemperature. However, even though the temperature is equal to or lowerthan the room temperature, it is possible to obtain the effect thereof.

If a pre-heat treatment is performed as described above, the aluminumshaped product where the networks of the crystallization products,acicular crystallization products, or the aggregates of crystallizationproducts formed during the continuous casting partially remain in thestructure even after forming and a heat treatment. The crystallizationproducts having these shapes resist against the deformation of analuminum matrix under high temperature. As a result, mechanical strengthis obtained under high temperature in the range of 250° C. to 400° C.That is, since the networks of the crystallization products, acicularcrystallization products, or the aggregates of crystallization productsresist against deformation under high temperature where the aluminummatrix is softened, aluminum shaped products have excellenthigh-temperature mechanical strength. Meanwhile, if a pre-heat treatmenttemperature is high and a percent reduction of the forging material ishigh, the networks of the crystallization products, acicularcrystallization products, or the aggregates of crystallization productsare divided and aggregated in a granular shape, and the aggregates in agranular shape are uniformly dispersed state in the aluminum matrixsoftening under high temperature. For this reason, the resistance of thecrystallization products against the deformation of the aluminum matrixunder high temperature deteriorates, and high-temperature mechanicalstrength is also not increased.

According to the present invention, under the above-mentioned alloycomposition, the aluminum matrix is softened, and the network oracicular crystallization products of crystallization products, oraggregates, which resist against the deformation of the aluminum matrix,partially remain in a high-temperature range higher than the range of250° C. to 400° C. where deformation occurs very easily, therebyincreasing high-temperature mechanical strength.

When a homogenization treatment is suppressed or omitted on a 6000series alloy or the like that is a dilute alloy where the amount ofcrystallization products is relatively small and the network or acicularcrystallization products of the crystallization products do not soappear, the suppression or omission of the homogenization treatmentfacilitates the suppression of recrystallization or the simplificationof steps, This is different from the present invention that facilitateshigh-temperature improvement by maintaining preferably the network oracicular crystallization products contained in a high-Si-content alloyforging material where the amount of crystallization products is largeand the network or acicular crystallization products appears duringcasting.

As described in the Background Art, the disclosure of Patent Document 1(Japanese Patent Application Publication No. 2002-294383) relates to a6000 series alloy, and the suppression or omission of the temperature ofthe homogenization treatment is performed not to obtain high-temperaturecharacteristics of the alloy but to improve mechanical characteristicsat normal temperature by suppressing recrystallization. The network oracicular crystallization products of the crystallization products doesnot so appear in the dilute alloy where the alloy system is alsodifferent and the amount of crystallization products is relativelysmall. Al—Mn and Al—Cr based compounds, which suppress therecrystallization, are finely precipitated by lowering and suppressingthe temperature of the homogenization treatment. This is different fromthe present invention that facilitates high-temperature improvement bymaintaining preferably the network or acicular crystallization productsin a high-Si-content alloy forging material where the amount ofcrystallization products is large and the network and acicularcrystallization products appear during casting.

In particular, in order to increase the high-temperature mechanicalstrength and improve the forgeability of the forging material, it ispreferable that the retention temperature of the pre-heat treatment bein the range of 200° C. to 370° C. If the retention temperature is setin this temperature range, it is possible to form an aluminum shapedproduct where the eutectic Si or intermetallic compounds at the time ofthe pre-heat treatment are hardly aggregated in a spherical shape, andthe networks of the crystallization products, acicular crystallizationproducts, or the aggregates of crystallization products formed duringthe continuous casting partially remain even after forging and apost-heat treatment, so that the aluminum shaped product has excellenthigh-temperature mechanical strength excellent.

In particular, in order to further increase the high-temperaturemechanical strength of the forging material, it is preferable that theretention temperature of the pre-heat treatment is in the range of −10°C. to 200° C. If the retention temperature is set in this temperaturerange, it is possible to form an aluminum shaped product where theeutectic Si or intermetallic compounds at the time of the pre-heattreatment are not almost aggregated in a spherical shape, and thenetworks of the crystallization products, acicular crystallizationproducts, or the aggregates of crystallization products formed duringthe continuous casting partially remain even after forging and apost-heat treatment, so that the aluminum shaped product has excellenthigh-temperature mechanical strength.

Further, in order to further increase the forgeability of the forgingmaterial, it is preferable that the retention temperature of thepre-heat treatment be in the range of the 370° C. to 480° C. If theretention temperature is set in this temperature range, it is possibleto form an aluminum shaped product where some entectic Si orintermetallic compounds at the time of the pre-heat treatment areaggregated in a spherical shape and the resistance against thedeformation is decreased during the casting, so that the aluminum shapedproduct has excellent forgeability. Furthermore, in this temperaturerange, it is possible to form an aluminum shaped product where thenetworks of the crystallization products, acicular crystallizationproducts, or the aggregates of crystallization products formed duringthe continuous forging partially remain even after the forging and apost-heat treatment, so that the aluminum shaped product has excellenthigh-temperature mechanical characteristics.

The pre-heat treatment step may be provided between after the castingand the forging step. For example, the pre-heat treatment step may beperformed within one day after the casting, or the forging material maybe provided to the forging step within one week after the pre-heattreatment step. Correction treatment and peeling treatment may beperformed during this period.

Next, an example of the forging step included in the present inventionwill be described. A method for production includes 1) a step of cuttingthe continuously cast round rod to a predetermined length, 2) a step ofpreliminarily heating and upsetting the cut forging material, 3) a stepof lubricating the upset forging material, 4) a step of providing theforging material into a mold so as to forge the forging material, and 5)a step of extracting product from the mold by a knock-out mechanism.

A lubricant may be applied to the forging material to be forged, and maybe heated before being provided to the upsetting treatment. Meanwhile,the upsetting step may be omitted.

A lubricant treatment may be the application of a water-solublelubricant or a bonde treatment. For example, it is preferable that theforging material be preliminarily heated at a temperature of 380° C. to480° C. and provided to a forging apparatus after the bonde treatment isperformed on the forging material. If the forging material ispreliminarily heated at a temperature of 380° C. to 480° C., thedeformability of the forging material is improved and easily formed in acomplicated shape.

It is preferable that an aqueous lubricant be used as the lubricant, andit is more preferable that a water-soluble graphite lubricant is used asthe lubricant. The reason for this is that graphite is easily seized onthe forging material. In this case, for example, it is preferable thatthe forging material is heated at a temperature of 380° C. to 480° C.and provided to a forging apparatus after a lubricant is applied to theforging material corresponding to a temperature of 70° C. to 350° C. andthen the forging material is cooled at normal temperature (for example,the forging material is retained for 2 to 4 hours). It is preferablethat an aqueous lubricant be used as the lubricant, and it is morepreferable that a water-soluble graphite lubricant be used as thelubricant. The reason for this is that graphite is easily seized on theforging material.

Before the forging material is provided, a lubricant is applied to thesurface of the mold. The amount of the lubricant may be furtherappropriately set in a state so as to correspond to the combination ofan upper mold and dies by adjusting a spraying time. It is preferablethat an oil-based lubricant be used as the lubricant. For example,mineral oil maybe used as the lubricant. The reason for this is that thetemperature of the mold may be lowered in the case of aqueous liquidlubricant but the lowering of the temperature can be suppressed. Since alubrication effect is improved if an oil-based lubricant is a mixture ofgraphite and mineral oil, it is more preferable that the oil-basedlubricant be used.

It is preferable that the heating temperature of the mold be in therange of 150° C. to 250° C. The reason for this is that a sufficientplastic flow can be obtained.

In the present invention, a percent reduction of a portion requiringhigh-temperature fatigue resistant strength is preferably 90% or less(preferably 70% or less) in the forging. Ifa percentreduction isequaltoor lessthan this percentreduction, it is possible to form a shapedproduct where the division of the networks of the crystallizationproducts, acicular crystallization products, or the aggregates ofcrystallization products is suppressed, so that the aluminum shapedproduct has excellent high-temperature mechanical strength.

Meanwhile, the portions of the shaped product, which requireshigh-temperature mechanical strength, may satisfy this percentreduction.

Meanwhile, if plastic forming step such as an upsetting step isperformed before forging, it is preferable that a percent reduction beconsidered as a total of the percent reductions of those plastic formingstops. For example, in case of the shaped product that have complicatedshapes, a percent reduction per processing is preferably in the range of10 to 80% (more preferably 10 to 50%) and processing is preferablyperformed several times (more preferably twice). For example, a percentreduction of the first processing is preferably in the range of 10 to50% (more preferably 10 to 30%).

Herein, a percent reduction is defined as follows.

Percent reduction=(thickness before plastic forming-thickness afterplastic forming)/(thickness before plastic forming)×100%

A post-heat treatment is performed on the resultant forged products. Thecombination of a solution treatment and an aging treatment may be usedas the post-heat treatment. The post-heat treatment may be performedwithin one week after the forging treatment.

Specifically, it is possible to perform a solution treatment on theforged products under conditions where the forged products are retainedat a temperature of, for example, 480° C. to 520° C. (preferably 490° C.to 510° C.) for 3 hours.

A T5 treatment or a T6 treatment of JIS standards may be performed onthe forged products as the post-heat treatment other than theabove-mentioned post-heat treatment.

In the present invention, it is preferable that the product taken out ofthe forging apparatus is retained at a temperature of 170° C. to 230° C.(more preferably 190° C. to 220° C.) for 1 to 10 hours as an agingtreatment without the solution treatment or quenching. It is possible toform a shaped product where the division and aggregation of the networksof the crystallization products, acicular crystallization products, orthe aggregates of crystallization products can be suppressed, whichmakes high-temperature mechanical strength excellent. Therefore, this ispreferable.

The alloy structure of the shaped product produced by theabove-mentioned method corresponds to aluminum the shaped product wherethe eutectic Si or intermetallic compounds are hardly aggregated in aspherical shape, and the networks of the crystallization products,acicular crystallization products, or the aggregates of crystallizationproducts formed during the continuous casting partially remain evenafter the forging and a post-heat treatment, so that the shaped productshas excellent high-temperature mechanical strength.

Further, the alloy composition contains 10.5 to 13.5% by mass(preferably, 11.5 to 13% by mass) of Si, 2.5 to 6% by mass (preferably,3.5 to 5.5% by mass) of Cu, 0.3 to 1.5% by mass (preferably, 0.5 to 1.3%by mass) of Mg, and 0.8 to 4% by mass (preferably, 1.8 to 3.5% by mass)of Ni, and corresponds to an aluminum-alloy that satisfies a relationalexpression of Ni (% by mass)≧[−0.68×Cu(% by mass)+AA(% by mass)](wherein, AA is a constant and AA≧4.2 preferably AA≧4.7).

It is preferable that the alloy composition contain one or two or moreof 0.1 to 1% by mass (preferably, 0.2 to 0.5% by mass) of Mn, 0.05 to0.5% by mass (preferably, 0.1 to 0.3% by mass) of Cr, 0.04 to 0.3% bymass (preferably, 0.1 to 0.2% by mass) of Zr, 0.01 to 0.15% by mass(preferably, 0.05 to 0.1% by mass) of V, and 0.01 to 0.2% by mass(preferably, 0.02% to 0.1% by mass) of Ti.

Further, itis preferablethat thealloy compositionincludes 0.15 to 0.65%by mass (preferably, 0.3 to 0.5% by mass) of Fe.

Furthermore, -it is preferable that the alloy composition includes 0.003to 0.02% by mass (preferably, 0.007 to 0.016% by mass) of P.

In addition, the alloy composition contains one or two or more of 0.003to 0.03% by mass (preferably, 0.01 to 0.02% by mass) of Sr, 0.1 to 0.35%by mass (preferably, 0.15 to 0.25% by mass) of Sb, 0.0005 to 0.015% bymass (preferably, 0.001 to 0.01% by mass) ofNa, and 0.001 to 0.02%bymass (preferably, 0.005 to 0.01% by mass) of Ca, which is preferablebecause there is an effect of micronizing primary Si crystals.

Examples

The present invention will be specifically described below by usingexamples. However, the present invention is not limited to theseexamples.

Examples 1 to 16 [Manufacturing Conditions]

The aluminum-alloy shaped product of Examples 1 to 16 shown in Table 1and Comparative Examples 1 to 10 shown in Table 2 were produced by aproduction system shown in FIG. 1.

TABLE 1 Temperature Percent Post- Fatigur Strength Stress of Homoge-reduction Heat (Unit: MPa) nization during the Treat- TemperatureTemperature Value Treatment course of ment Composition of Aluminum-alloy(% by mass) Condition Condition of (° C.) upsetting (T6, T5) Si Fe Cu MnMg Ni Ti P Sr 300° C. 350° C. AA Example 1 370 50% T6 10.5 0.25 2.7 —0.95 3.8 — — 0.015 60 45 5.64 Example 2 370 50% T6 10.5 0.25 2.7 — 0.953.8 — 0.015 — 59 44 5.64 Example 3 370 50% T6 12.8 0.48 3.0 0.23 0.953.0 0.075 0.018 — 59 43 5.04 Example 4 370 50% T5 12.8 0.48 3.0 0.230.95 3.0 0.075 0.018 — 62 44 5.04 Example 5 370 50% T6 11.8 0.33 3.2 —0.72 2.2 — 0.005 — 54 39 4.38 Example 6 370 50% T6 12.8 0.25 3.8 — 0.951.8 — 0.018 — 53 38 4.38 Example 7 370 50% T6 13.4 0.25 4.1 — 1.10 2.2 —0.018 — 57 43 4.99 Example 8 370 50% T6 13.4 0.61 4.1 0.32 1.21 2.2 —0.010 — 58 43 4.99 Example 9 not over 200 50% T6 13.4 0.61 4.1 0.32 1.212.2 — 0.010 — 59 44 4.99 Example 10 370 50% T6 12.8 0.48 4.5 0.23 0.951.5 0.075 0.018 — 55 40 4.56 Example 11 370 50% T6 12.5 0.28 5.1 0.211.14 1.1 — 0.007 — 55 39 4.57 Example 12 370 50% T6 12.8 0.25 5.5 — 0.951.0 — 0.018 — 57 43 4.74 Example 13 370 50% T6 12.8 0.48 5.5 0.23 0.951.0 0.075 0.018 — 58 44 4.74 Example 14 370 50% T6 10.5 0.25 5.7 — 0.953.5 — 0.010 — 62 47 7.38 Example 15 370 88% T6 12.8 0.48 3.0 0.23 0.953.0 0.075 0.018 — 58 41 5.04 Example 16 470 50% T6 12.8 0.48 3.0 0.230.95 3.0 0.075 0.018 — 58 41 5.04

TABLE 2 Temperature Percent Post- Fatigur Strength Stress of Homoge-reduction Heat (Unit: MPa) nization during the Treat- TemperatureTemperature Value Treatment course of ment Composition of Aluminum-alloy(% by mass) Condition Condition of (° C.) upsetting (T6, T5) Si Fe Cu MnMg Ni Ti P Sr 300° C. 350° C. AA Comparative 370 50% T6 11.0 0.25 3.00.10 0.40 1.8 — 0.010 — 45 30 3.84 Example 1 Comparative 370 50% T6 12.30.3 3.3 0.15 0.85 1.8 0.05 0.005 — 47 32 4.04 Example 2 Comparative 47050% T6 12.3 0.3 3.3 0.15 0.85 1.8 0.05 0.005 — 45 30 4.04 Example 3Comparative 370 50% T6 12.8 0.48 4.0 — 0.95 1.2 — 0.010 — 46 31 3.92Example 4 Comparative 370 50% T6 12.8 0.48 5.0 — 0.95 0.5 — 0.010 — 4632 3.90 Example 5 Comparative 500 50% T6 13.4 0.61 4.1 0.32 1.21 2.2 —0.010 — 48 35 4.99 Example 6 Comparative 370 50% T6 12.3 0.3 5.7 0.160.98 0.5 — 0.010 — 49 36 4.38 Example 7 Comparative 370 50% T6 12.4 0.36.3 0.17 0.97 0.6 — 0.010 — *1 *1 4.88 Example 8 Comparative 370 50% T612.3 0.32 2.3 0.16 0.94 3.7 0.05 0.010 — 49 35 5.26 Example 9Comparative 370 50% T6 12.4 0.36 2.3 0.15 0.99 4.3 — 0.010 — *1 *1 5.86Example 10

Continuously cast round rods, which each have a diameter of φ85 (mm) andare made of aluminum-alloys of Examples 1 to 16 having a compositionshown in Table 1 and Comparative Examples 1 to 10 shown in Table 2, werecast by using a hot top continuous casting apparatus shown in FIG. 5 asthe continuous casting apparatus 81 configuring the production system.The hot top continuous casting apparatus is a caster using a gaspressurization hot top casting method, and is configured so that gas andliquid lubricant are introduced into a clearance between a header and amold and the pressure of the molten alloy supplied to the mold, liquidlubricant, and gas obtained from the vaporization of the liquidlubricant are preferably balanced with each other. Since an area wherethe molten aluminum comes in contact with the mold is small due to thisconfiguration, it is possible to rapidly cool and solidify a moltenalloy by cooling water and to stably cast a continuously cast rod madeof an aluminum-alloy.

After that, as the pre-heat treatment step, a homogenization treatmentwas performed on each of the continuously cast round rods attemperatures shown in Tables 1 and 2. Each of the continuously castround rods was cut at a thickness of 20 or 80 mm and was used as aforging material to be forged. Then, after forging materials to beforged were preliminarily heated at a temperature of 420° C., eachupsetting step was performed at predetermined percent reductions duringthe course of upsetting shown in Tables 1 and 2 and plastic forming wasperformed in a predetermined shape.

Meanwhile, when an upsetting step was performed at a percent reductionduring the course of upsetting of 55% on Examples 5 to 7 and 10 to 13, acrack rate was evaluated. The evaluation results are shown in Table 3.In Table 3, an ◯ mark indicated that a crack rate caused by an upsettingstep was less than 1%, and a Δ mark indicated that a crack rate causedby an upsetting step was equal to or larger than 1%.

TABLE 3 Temperature of Percent reduction Homogenization during theTreatment course of Content in Aluminum-alloy (wt %) Value of Crack (°C.) upsetting Cu Ni AA Rate Example 5 370 55% 3.2 2.2 4.38 Δ Example 6370 55% 3.8 1.8 4.38 ◯ Example 7 370 55% 4.1 2.2 4.99 Δ Example 10 37055% 4.5 1.5 4.56 ◯ Example 11 370 55% 5.1 1.1 4.57 ◯ Example 12 370 55%5.5 1.0 4.74 ◯ Example 13 370 55% 5.5 1.0 4.74 ◯

After that, each of Examples and Comparative Examples was produced byperforming a predetermined post-heat treatment step shown in Tables 1and 2 on the forging material on which plastic forming was has beenperformed.

Meanwhile, the post-heat treatment step was performed by any one of a T5treatment that quenched plastic worked articles with water and retainedthe plastic worked articles at a temperature of 210° C. for 6 hours; anda T6 treatment that retained plastic worked articles at a temperature of500° C. for 2.5 hours, quenched the plastic worked articles with water,and retained the plastic worked articles at a temperature of 210° C. for6 hours.

[Evaluation of Fatigue Strength]

The fatigue strength of each of Examples and Comparative Examples wasevaluated by the following method.

Test pieces were fabricated from each of Examples and ComparativeExamples, and the fatigue strength of each of the test pieces wasevaluated under environment of 300° C. and 350° C. by an Ono-type rotarybending fatigue testing machine after the test pieces were preliminarilyheated at a temperature of 300° C. or 350° C. for 100 hours. Repeatedstress was applied 10,000,000 times, and stress where the test piece wasnot broken was measured.

Tables 1 and 2 show the composition, the heat treatment condition, thepercent reduction during the course of upsetting, and the evaluationresult of fatigue strength of each of Examples and Comparative Examples,and a constant AA that satisfies a relational expression defined by Ni(%by mass)=[−0.68×Cu(% by mass)+AA(% by mass)]. Further, FIG. 6 shows arelationship between the percentage contents of Ni and Cu in thecomposition of each of Examples and Comparative Examples. Meanwhile, inFIG. 6, the respective values of AA of Examples 1 to 14 were representedby reference characters S1 to S14, respectively, and the respectivevalues of AA of Comparative Examples 1 to 10 (excluding ComparativeExample 6) were represented by reference characters C1 to C10,respectively.

All Examples 1 to 16 were produced by the method for productionaccording to the present invention, and have fatigue strength of 33 MPaor more at a temperature of 350° C. as appreciated from Table 1. Sincehaving target fatigue strength as described above, Examples 1 to 16produced by the method for production according to the present inventionmay be preferably used for-parts that require mechanical strength athigh temperature.

It is essential for the aluminum-alloy, which is used in S the methodfor production according to the present invention, to have thecomposition where Ni content and Cu content are included in a regionsurrounded by A-B-C-D-E-A of FIG. 6.

All Examples 10 to 13 and Example 6, of which Ni content and Cu contentare included in a region surrounded by D-E-H-I-D, can be processed overan percent reduction during the course of upsetting of 55% asshowninTable 3. Thus, inthe presentinvention, it is more preferable to use analuminum-alloy containing Cu content so that Ni content is equal to orless than 2.0 wt % and AA≧4.2 is satisfied.

In contrast, Comparative Examples 1-to 5 and 7 to 10, which havecomposition out of the range of the alloy composition defined in themethod for production according to the present invention, did not havetarget fatigue strength as shown in Table 2. Comparative Examples 8 and10 had poor plastic workability and generated cracks during upsetting.“*1” shown in Table2 indicates a case that a test piece of ComparativeExample cannot have been sampled. Meanwhile, the values of AA ofComparative Examples 1 to 4 were less than 4.2. Further, ComparativeExample 6, on which a pre-heat treatment step was performed at atemperature out of the temperature range defined in the method forproduction according to the present invention, also did not have targetfatigue strength.

[Evaluation of Metal Structure]

Samples of which structure to be observed were cut out from a centerportion of a vertical cross section of each of Examples of Table 1 andComparative Examples of Table 2, and the samples were micro-polished.Then, the networks of the crystallization products of the samples wereobserved from microphotographs of the samples in order to evaluate themetal structure of each of Examples and Comparative Examples.

It could be confirmed that the networks of the crystallization products,acicular crystallization products, or the aggregates of crystallizationproducts formed during the continuous casting partially remain in thestructure of Examples even after forming and a heat treatment.

Further, as for each of the Examples, an area occupation ratio ofeutectic Si is 8% or more, an average grain size of the eutectic Si is 5μm or less, and the eutectic Si of an acicular ratio of 1.4 or more is25% or more; and an area occupation ratio of an intermetallic compoundis 1.2% or more, an average grain size of an intermetallic compound of1.5 μm or more. And a length of an intermetallic compound or a length ofan aggregate of a contacted intermetallic compound of 30% or more is 3μm or more.

In particular, as shown in Table 4, all Examples 10 and 13, whichcontain Ni and Co at preferred concentration, have average grain sizesof eatectic Si of 2.5 μm or less. It is appreciated that both Examples10 and 13 have about 80% eutectic Si of which acicular ratios are 1.4 ormore, and have about 90% ormore aggregates of intermetallic compounds ofwhich length is 3 μm or more.

Further, according to the results of Tables 1 and 4, it is appreciatedthat Example 13 having a constant AA larger than 4.7 has a larger amountof network-like or acicular intermetallic compounds contributing tohigh-temperature strength, and higher fatigue strength as compared toExample 10 having a constant AA less than 4.7. As described above, inthe present invention, the aluminum-alloy shaped product prepared aconstant AA of 4.7 or more are preferable.

In contrast, each of comparative Examples had a smaller percentagecontent of eutectic Si having an acicular ratio of 1.4 or more, and asmaller length of an intermetallic compound or a smaller length of anaggregate of a contacted intermetallic compound, as compared toExamples. For example, as shown in Table 4, Comparative Example 6included only about 22% eutectic Si of which acicular ratio is 1.4 ormore. And an intermetallic compound or an aggregate of a contactedintermetallic compound of which length is 3 μm or more is only about 28%in Comparative Example 6.

TABLE 4 Eutectic Si Intermetallic Compound Area Average Acicular AreaAverage Acicular Occupation Grain Ratio of Occupation Grain Ratio ofRatio (%) Size 1.4 or More Ratio (%) Size 1.4 or More Example 10 8.6%2.4 μm 78% 7.4% 2.6 μm 88% Example 13 8.5% 2.5 μm 80% 7.8% 2.7 μm 89%Comparative 8.5% 2.0 μm 22% 7.2% 1.9 μm 28% Example 6

Examples 17 and 18 [Manufacturing Conditions]

Examples 17 and 18 and Comparative Examples 11 and 12, respectively,were produced under the composition and manufacturing conditions shownin Table 5 by the same method for production as Examples 1 to 16 andComparative Examples 1 to 10.

Meanwhile, Comparative Example 13 was made of a powdery extruded-castmaterial, and was produced by the same method for production asComparative Examples 11 and 12 except that Comparative Example 13 wasnot formed from a continuously cast round rod made of an aluminum-alloyand a homogenization treatment was not performed. All Examples 17 and 18and Comparative Examples 11 to 13 were formed as the aluminum-alloyshaped product having the shape of a piston 1 that had a diameter of 80mm and a top surface 10 having a thickness of 8 mm as shown in FIGS. 7Ato 7C.

[Evaluation of Fatigue Strength]

The fatigue strength of each of Examples 17 and 18 and ComparativeExamples 11 to 13 was evaluated by the following method.

First, after the piston 1 of each of Examples and Comparative Exampleswas preliminarily heated at a temperature of 300° C. or 350° C. for 100hours, a test piece 11 was cut out from a center portion of the topsurface 10 of each of Examples and Comparative Examples. The fatiguestrength of each of the test pieces 11 was evaluated by a pulsatingtensile fatigue test under temperature environment corresponding to thepreliminary heating temperature. In the fatigue test, a stress ratio Rwas −0.1, and the maximum stress where the test piece was not brokenagainst the application of stress 10,000,000 times was referred to asfatigue strength. Table 5 shows the evaluation results of the fatiguestrength of Examples 17 and 18 and Comparative Examples 11 to 13.

As appreciated from Table 5, the fatigue strength of Examples 17 and 18at a temperature of 350° C. exceeds 43 MPa that is preferable for a partrequiring mechanical strength at high temperature, and the fatiguestrength thereof at a temperature of 300° C. exceeds 55 MPa. Further,since Examples 17 and 18 correspond to Examples 10 and 13 where the samemanufacturing conditions as Examples 17 and 18 except for shapes areused, it is appreciated that Examples 17 and 18 have stable mechanicalstrength a thigh temperature despite an evaluation method.

TABLE 5 Temperature Post- Fatigur Strength Stress of Homoge- Heat (Unit:MPa) nization Treat- Temperature Temperature Value Treatment mentComposition of Aluminum-alloy (% by mass) Condition Condition of Forgingmaterial (° C.) (T6, T5) Si Fe Cu Mn Mg Ni Ti P 300° C. 350° C. AAComparative Continuously 370 T6 12.3 0.3 3.3 0.15 0.85 1.8 0.05 0.005 6445 4.04 Example 11 Cast Rod Comparative Continuously 370 T6 12.4 0.3 1.0— 1.04 1.0 — 0.010 45 33 1.66 Example 12 Cast Rod Comparative Powdery —T6 11.7 5.3 2.5 — 1.1 — — — 80 59 1.70 Example 13 Extruded-Cast MaterialExample 17 Continuously 370 T6 12.8 0.48 4.5 0.23 0.95 1.5 0.075 0.01870 52 4.56 Cast Rod Example 18 Continuously 370 T6 12.8 0.48 5.5 0.230.95 1.0 0.075 0.018 73 54 4.74 Cast Rod

In contrast, a value of AA of Comparative Example 11 is less than 4.2,and corresponds to Comparative Example 2 where the same manufacturingconditions as Comparative Example 11 except for shapes are used. Fromthe evaluation results of the fatigue strength of Comparative Example 2of Table 2 and Comparative Example 11 of Table 5, it is considered thatthe reliability of the mechanical strength of Comparative Example 11lacks at high temperature.

Further, AA of Comparative Example 12 is 1.68, and the fatigue strengththereof at a temperature of 350° C. is significantly lower than 43 MPa.

Meanwhile, Comparative Example 13 made of a powdery extruded-castmaterial has fatigue strength higher than the fatigue strength ofExamples 17 and 18, regardless of a fact that AA is 1.7. However, thereis a drawback in that a fine portion, for example, a skirt portion 12 ofa sample formed by packing is apt to become brittle. Thus, the shapedproduct using the powdery extruded-cast material have poorer ductilityand toughness as compared to the aluminum-alloy shaped products thatinclude a forging step using a continuously cast rod made of analuminum-alloy as forging material.

Since having excellent ductility, toughness, and fatigue strength, thealuminum-alloy shaped product, which are produced by the method forproduction according to the present invention, may be preferably usedfor top surfaces or the like of a piston of an internal combustionengine.

INDUSTRIAL APPLICABILITY

As described above, the present invention provides a method forproduction of aluminum-alloy shaped product that includes a forging stepusing a continuously cast rod made of an aluminum-alloy as forgingmaterial. The aluminum-alloy contains Si, Cu, Mg, and Ni. Accordingly,according to the present invention, it is possible to obtain a shapedproduct that has excellent high-temperature fatigue strength,forgeability, ductility, and toughness, Further, in Ni and Cu, since arelational expression of “Ni(% by mass)≧[−0.68×Cu(% by mass)+4.2(% bymass)] is satisfied, it is possible to further improve fatigue strengthcharacteristics at high temperature.

It is possible to further reduce the thickness of a piston of aconventional internal combustion engine by using the aluminum-alloyshaped product according to the present invention and to reduce theweight of a piston of an internal combustion engine. Further, it ispossible to satisfy weight reduction required from the market, to reducefuel consumption of an internal combustion engine, and to improveoutput.

1. A method for producing an aluminum-alloy shaped product, comprising:a step of forging a continuously cast rod of aluminum-alloy serving as aforging material, in which the aluminum-alloy contains Si in an amountof 10.5 to 13.5 mass %, Cu in an amount of 2.5 to 6 mass %, Mg in anamount of 0.3 to 1.5 mass % and Ni in an amount of 0.8 to 4%, andsatisfies a relational expression of “Ni(% by mass)≧(−0.68×Cu(% bymass)+4.2(% by mass)), and heat treatment and heating steps including astep of subjecting the forging material to pre-heat treatment, a step ofpreliminary heating the forging material before a course of forging ofthe forging material and a step of subjecting a shaped product topost-heat treatment, said pre-heat treatment including treatment ofmaintaining the forging material at a temperature of −10 to 480° C. fortwo to six hours.
 2. The method according to claim 1, wherein thepre-heat treatment is performed at a temperature of at least 200° C. and370° C. or lower.
 3. The method according to claim 1, wherein thepre-heat treatment is performed at a temperature of at least −10° C. andless than 200° C.
 4. The method according to claim 1, wherein thepre-heat treatment is performed at a temperature of at least 370° C. and480° C. or lower.
 5. The method according to claim 1, wherein thepost-heat treatment is performed at 170 to 230° C. for one to 10 hourswithout performing solid solution treatment.
 6. The method according toclaim 1, wherein, the aluminum-alloy further contains Fe in an amount of0.15 to 0.65 mass %.
 7. The method according to claim 1, wherein thealuminum-alloy further contains P in an amount of 0.003 to 0.02 mass %.8. The method according to claim 1, wherein the aluminum-alloy furthercontains at least one species selected from among Sr in an amount of0.003 to 0.03 mass %, Sb in an amount of 0.1 to 0.35 mass %, Na in anamount of 0.0005 to 0.015 mass % and Ca in an amount of 0.001 to 0.02mass %.
 9. The method according to claim 1, wherein the aluminum-alloyfurther contains at least one species selected from among Mn in anamount of 0.1 to 1.0 mass %, Cr in an amount of 0.05 to 0.5 mass %, Zrin an amount of 0.04 to 0.3 mass %, V in an amount of 0.01 to 0.15 mass% and Ti in an amount of 0.01 to 0.2 mass %.
 10. The method according toclaim 1, wherein during the forging step, a percent reduction of aportion of the forging material that requires high-temperature fatiguestrength resistance is regulated to 90% or less.
 11. The methodaccording to claim 1, wherein in the forging step, the preliminaryheating step is performed at a temperature of 380 to 480° C.
 12. Themethod according to claim 1, wherein the continuously cast rod isproduced through continuous casting of a molten aluminum-alloy having anaverage temperature which falls within a range of a liquidus temperature+40° C. to the liquidus temperature +230° C. at a casting speed of 80 to2,000 mm/minute.
 13. An aluminum-alloy shaped product produced throughthe method according to claim 1 and having a metallographic structure inwhich crystallization product networks, acicular crystallizationproducts or crystallization product aggregates that have been formedduring a course of continuous casting remain partially even afterforging and heat treatment steps.
 14. An aluminum-alloy shaped productproduced through the method according to claim 1 and having a eutecticSi area share of 8% or more, an average eutectic Si particle diameter of5 μm or less, 25% or more of eutectic Si having an acicular eutectic Siratio of 1.4 or more, an intermetallic compound area share of 1.2% ormore, an average intermetallic compound particle diameter of 1.5 μm ormore and 30% or more of intermetallic compounds or intermetalliccompound aggregates having an intermetallic compound length orintermetallic compound aggregate length of 3 μm or more.
 15. Analuminum-alloy shaped product produced through the method according toclaim 13, wherein an engine piston is made of the aluminum-alloy andincludes a top surface portion and a skirt portion and thehigh-temperature fatigue strength of the top surface portion is 50 MPaor more.
 16. A production system comprising a continuous line forperforming a series of steps for producing an aluminum-alloy shapedproduct from a molten aluminum-alloy, wherein the series of stepsincludes at least the steps of the method of claim 1.